1. Field of the Invention
This invention relates to a new process for the production of low-monomer urethane- or isocyanurate-modified polyisocyanates based on 2,4-diisocyanatotoluene or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane by the selective reduction of the content of these monomers in the modified polyisocyanates by the addition of water.
2. Description of the Prior Art
It is known (DE-OS No. 2,414,413 and DE-OS No. 2,414,391) that the content of monomeric starting diisocyanate in isocyanurate- or urethane-modified polyisocyanates can be reduced by the addition of monohydric alcohols, a reaction taking place between the alcohol added and the monomeric starting diisocyanates to be removed. However, this process is attended by the disadvantage that the monohydric alcohol reacts not only with the monomeric starting diisocyanate, but also partially with the modified polyisocyanate, so that its NCO functionality is reduced.
It has now surprisingly been found that through measured addition of water, the reaction between monomeric starting diisocyanate and water, particularly in the case of 2,4-diisocyanatotoluene, takes place more selectively than when a monohydric or dihydric alcohol is added. The result of this (plus the fact that water is a difunctional compound in the context of the isocyanate addition reaction) is that, through the addition of water, the monomer content is reduced far more selectively without any adverse effect on the NCO functionality of the modified polyisocyanates.